1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
One of the most challenging requirements for micro-lithography for the production of integrated circuits as well as liquid crystal display panels is the positioning of tables. For example, sub-100 nm lithography demands substrate- and mask-positioning stages with dynamic accuracy and matching between machines to the order of 1 nm in all 6 degrees of freedom (DOF), at velocities of up to 3 m/s.
A popular approach to such demanding positioning requirements is to sub-divide the stage positioning architecture into a coarse positioning module (e.g. an X-Y table or a gantry table) with micrometer accuracies but travelling over the entire working range, onto which is cascaded a fine positioning module. The latter is responsible for correcting for the residual error of the coarse positioning module to the last few nanometers, but only needs to accommodate a very limited range of travel. Commonly used actuators for such nano-positioning include piezoelectric actuators or voice-coil type electromagnetic actuators. While positioning in the fine module is usually effected in all 6 DOF, large-range motions are rarely required for more than 2 DOF, thus easing the design of the coarse module considerably.
The micrometer accuracy required for the coarse positioning can be readily achieved using relatively simple position sensors, such as optical or magnetic incremental encoders. These can be single-axis devices with measurement in one DOF, or more recently multiple (up to 3) DOF devices such as those described by Schaffel et al “Integrated electro-dynamic multicoordinate drives”, Proc. ASPE Annual Meeting, California, USA, 1996, p. 456-461. Similar encoders are also available commercially, e.g. position measurement system Type PP281R manufactured by Dr. J. Heidenhain GmbH. Although such sensors can provide sub-micrometer level resolution without difficulty, absolute accuracy and in particular thermal stability over long travel ranges are not readily achievable.
Position measurement for the mask and substrate tables at the end of the fine positioning module, on the other hand, has to be performed in all 6 DOF to sub-nanometer resolution, with nanometer accuracy and stability. This is commonly achieved using multi-axis interferometers to measure displacements in all 6 DOF, with redundant axes for additional calibration functions (e.g. calibrations of interferometer mirror flatness on the substrate table).
However, with the above approach, every time the stage is brought (back) into the range of the fine positioning module, the position of the stage has to be (re)calibrated in six degrees of freedom. This takes a considerable amount of time, and as a result the throughput of the lithographic apparatus is decreased.
As an alternative for interferometers it is known to use optical encoders, possibly in combination with interferometers. Such optical encoders are for instance disclosed in US 2004/0263846 A1, which document is hereby incorporated herein by reference. The optical encoders described in this application make use of a grid plate that comprises a grid pattern which is used to determine the position of a sensor with respect to the grid pattern. In an embodiment the sensor is mounted on the substrate table and the grid plate is mounted on a frame of the lithographic apparatus.
However, the sensor range of such sensor is in principle limited to the size of the grid plate. However, the size of such grid plate is physically limited due to the high resolution required in the grid. Thus, in practice the size of the working area of such sensor is limited. Furthermore, it is possible that holes/openings have to be provided in the gridplate, for instance an opening through which the projection beam can be brought. At the location of such hole/opening, a sensor cannot determine its position. Moreover, it is possible that the grid plate is locally damaged, which may make an accurate determination of the position of a sensor on this position impossible.